In the future many of the advanced low signature vehicles, such as air planes, missiles, unmanned aerial vehicles (UAV), ships and terrain vehicles, will be equipped with some kind of array antenna. It is of great importance that these array antennas exhibit low passive radar cross section.
It is a known fact that array antennas may cause a very high radar cross section. The total radar cross section of an array antenna is the result of several subcontributions. The most important subcontributions are mirror reflection, edge scattering, scattering, reflections in the feed network, grating lobes, scattering caused by the location of the antenna elements in the aperture and diffuse scattering due to mechanical inaccuracy of manufacture. For hull integrated antennas the antenna behaves electromagnetically different than the surrounding hull and in particular within the frequency band of operation of the antenna. The transition between the antenna and the surrounding hull consists of an impedance transition causing scattering and due to that radar cross section. Accordingly, the material of the surrounding hull may be of great significance.
Prior art array antennas of today are commonly designed based upon given requirements on antenna performance, such as frequency of operation, band width, field of view, lobe widths, side lobe level and polarisation. An example of an array antenna designed based upon such requirements is known from U.S. Pat. No. 6,323,809 disclosing designing of a fragmented array antenna. When designing array antennas in this way the signature reduction is set aside and has to be considered afterwards when mounted in a hull. One way of obtaining signature reduction in this connection is to introduce frequency selective surfaces and space demanding absorbents located around the edges of the array antenna. One disadvantage of frequency selective surfaces is that they perform insufficient with respect to signature reduction for frequencies and polarisation coinciding with the frequency and polarisation of the antenna. Furthermore, if the surface is curved it may be difficult to design and manufacture frequency selective surfaces.
The hulls of future low signature air vehicles will most likely consist of some kind of composite material. Such material does not behave as conducting metals having very good conductivity. Furthermore the conductivity of composites may be anisotropic, i.e. the conductivity varies in different directions. A frequency selective surface usually behaves electromagnetically as a good electric conductor within its suppressed frequency band. If the surrounding material consists of a composite the hull and the frequency selective surface will behave electromagnetically different and due to that be the cause of radar cross section.